The present invention relates to a method for predicting dynamic behavior characteristics of a vehicle using screw theory.
It is quite difficult to exactly predict vehicle motion characteristics. However, these characteristics are very important factors that may affect a degree of a driver""s fatigue, and the drivability and stability of a vehicle. Therefore, when developing a new vehicle, a design target for such vehicle motion characteristics is predetermined, and overall geometry of a vehicle is determined such that the predetermined target can be achieved.
If vehicle development is performed without a prediction of vehicle motion characteristics, development through trial and error is required. This substantially increases development costs and the probability of failure. Therefore, during vehicle development, a suspension system is first designed and its performance estimated before manufacturing the actual vehicle. This development process is repeated until the target criteria have been met. Subsequently, the vehicle is manufactured and the performance of the suspension system tested in actual vehicle tests.
Various conventional methods for estimating the performance of the suspension system have been developed. However, such conventional methods have many drawbacks. For example, typically the front suspension geometry and the rear suspension geometry are independently designed. The performance of the suspension is then optimized by regulating tuning elements of the front suspension system and the rear suspension system.
Furthermore, roll behavior of a vehicle is affected by a relative change between the front and rear wheel suspensions. Therefore, it is difficult to optimize roll behavior of a vehicle through respective estimation of the front wheel suspension and the rear wheel suspension.
In light of the above, it would be desirable to provide a method for predicting roll behavior of a vehicle by simultaneously estimating front wheel suspension and rear wheel suspension changes.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.
Therefore, a motivation of the present invention is to provide dynamic behavior characteristics of a vehicle, using parameters acquired by a quasi-static analysis and change rates thereof, so that driving safety efficiency can be predicted.
In a preferred embodiment of the present invention, the method for predicting dynamic behavior characteristics of a vehicle suspension comprises: developing a vehicle model for a quasi-static analysis; performing the quasi-static analysis for the vehicle model under a cornering condition in which a specific lateral force acts; determining a finite screw axis based on a rigid body displacement of the vehicle model with respect to the ground through the quasi-static analysis; determining a fixed screw axis surface formed by a migration of the finite screw axis; calculating gradients of screw parameters with respect to the lateral force when the vehicle model behaves in an initial cornering state; and estimating roll behavior based on the fixed screw axis surface and gradients of screw parameters.
It is preferable that a body of the vehicle model is coupled to a contact patch through a spring such that a tire displacement caused by a vertical load can be expressed, wherein the contact patch is configured such that there is no vertical movement, and an equilibrium of lateral forces acting on the vehicle model exists without a structural restraint in a lateral direction.
Preferably, the equilibrium of lateral forces is realized by applying a lateral force that increases at a predetermined rate at a center of gravity of the vehicle model, and by applying corresponding lateral forces to the contact patch.
It is preferable that the vehicle model is structurally restrained in a forward/rearward direction to facilitate determination of a moment equilibrium according to a force equilibrium.
It is also preferable that a front wheel is configured to be restrained and a rear wheel is configured to undergo a forward/rearward movement such that a wheel base change that is brought about during roll behavior by a forward/rearward movement of the contact patch caused by geometries of the front and rear wheel can be reflected in the analysis.
Preferably, a forward/rearward displacement of a front contact patch is measured while bumping and rebounding the vehicle in a state whereby all forward/rearward restraints are removed, and a value obtained by a sine function approximation is used as a front wheel displacement restraint condition.
It is preferable that in the quasi-static analysis, when comparing a plurality of results of the quasi-static analysis for different vehicle models, a camber change tendency and a toe change tendency are set as similar values between the models, and a roll center at an initial position is set to be the same in each model.
It is preferable that in the step of estimating roll behavior, a point where the fixed screw axis intersects a surface that passes through a center of a front wheel and is perpendicular to a vehicle driving direction is considered as a roll center of the front wheel, a point where the fixed screw axis insects a surface that passes through a center of gravity is considered as a roll center of the center of gravity, and a point where the fixed screw axis insects a surface that passes through a center of a rear wheel is considered as a roll center of the rear wheel, and wherein a tendency of change of the roll center at the three points is estimated.
It is also preferable that the screw parameters include a first position parameter relating to a vertical migration of a center of gravity of the vehicle model, a second position parameter relating to a lateral migration of a center of gravity of the model, a first direction parameter relating to a pitch motion of the vehicle model, a second direction parameter relating to a yaw motion of the vehicle model, and a pitch parameter relating to cornering speed.
In another preferred embodiment of the present invention, the method for predicting dynamic behavior characteristics of a vehicle comprises: determining screw parameters and gradients of the screw parameters of a fixed screw axis through an analysis of a quasi-static vehicle model; and estimating roll behavior of the vehicle based on the determined screw parameters and the gradients of the screw parameters.
It is preferable that said determining comprises: applying a lateral force that increases at a predetermined rate at a center of gravity of the quasi-static vehicle model, and simultaneously applying corresponding lateral forces at each contact patch of the quasi-static vehicle model, so that a force equilibrium in a lateral direction and a moment equilibrium exist; and determining the screw parameters and the gradients of the screw parameters of a fixed screw axis based on motions of the quasi-static vehicle model with respect to the ground.
It is also preferable that the screw parameters include a lateral position parameter that relates to a lateral position of the fixed screw axis, and a center of gravity of a vehicle is estimated to lower if a value of the gradient of the lateral position parameter is negative, and a center of gravity of a vehicle is estimated to rise if a value of the gradient of the lateral position parameter is positive.
Preferably, the screw parameters include a vertical position parameter that relates to a vertical position of the fixed screw axis, and it is estimated that a lateral migration of a center of gravity of the vehicle decreases and a roll angle decreases if a value of a gradient of the vertical position parameter is negative.
It is preferable that the screw parameters include a lateral direction component of a unit direction vector of the screw axis, and it is estimated that a vehicle body slants in a forward direction if a value of a gradient of the lateral direction component is negative, and the vehicle body slants in a rearward direction if a value of a gradient of the lateral direction component is positive.
Preferably, the screw parameters include a vertical direction component of a unit direction vector of the screw axis, and a yaw behavior is estimated based on a gradient of the vertical direction component.
It is preferable that the screw parameters include a pitch parameter relating to a translating motion of a vehicle, and it is estimated that the vehicle moves in a forward direction along the screw axis if a value of a gradient of the pitch parameter is positive, and the vehicle moves in a rearward direction along the screw axis if a value of a gradient of the pitch parameter is negative.
It is further preferable that the quasi-static vehicle model comprises a body, and a contact patch that is coupled to the body through a spring, wherein the contact patch is configured to have no structural restraint in a lateral direction.